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Quantum crypto reaches 150 km

12 Mar 2004

A single photon is sent over a 150 km optical link beating the previous transmission record by 50 km.

Scientists at NEC in Japan claim to have smashed the transmission distance record for quantum cryptography. The team says it successfully sent a single photon over a 150-km-long optical fiber link. This significantly extends the previous record of 100 km, which was announced in June 2003.

Quantum cryptography uses a stream of single photons to transfer a secret key between a transmitter and a receiver. Each transmitted bit of the cryptographic key is encoded upon a single photon. Any attempt to intercept the key changes the quantum state of the photons, which reveals the presence of a hacker.

NEC’s record-breaking system relies on planar lightwave circuit (PLC) technology and a low-noise photon receiver. The system was developed by a collaboration of researchers from NEC, the Telecommunications Advancement Organization of Japan and the Japan Science and Technology Agency.

According to NEC, its system has two distinct benefits. Firstly, it offers stable one-way photon transmission. This reduces the noise of backscattered photons from the optical fiber to below one-tenth that of conventional round-trip systems.

The system’s second plus-point is an alleged ten-fold increase in signal-to-noise ratio compared with current systems. This is largely thanks to the receiver’s increased sensitivity to photons that have been broadened by dispersion in the long fiber-optic link.

“Due to wide-area coverage, this system can realize quantum cryptography transmissions in optical network in metropolitan areas and is expected to contribute to the realization of an optical fiber network system requiring advanced safety levels to prevent code-breaking in the future,” said NEC in its press release announcing the breakthrough.

Jacqueline Hewett is technology editor on Optics.org and Opto & Laser Europe magazine.

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